Electric furnace resistor



-lllllllllllllll il m m L Fll Jan. 19, 1932. J. KELLEHER ELECTRICFURNACE RESISTOR Filed Nov. 27, 1925 I avwmwtoz Jame; hflllakar 3511 W511 1014404 4 *Bowoc Patented Jan. 19, 1932 UNITED STATES PATENT OFFICEJAMES KELLEHER, OF CHIPPAWA, ONTAEBJO, CANADA, ASSIGNOR TO HARPERELECTRIC FURNACE CORPORATION, A. CQRYORATION OF NEW YORK ELECTRICFURNACE RESISTOR Application filed November 27, 1925. Serial No. 71,516.

This invention relates to a resistor adapted for use as a heatingelement of an electric furnace, and to the method of forming same.

Among the objects of my invention is to produce a resistor of thischaracter which is highly eflicient in operation, strong and durable inuse, and which may be operated at a high temperature withoutdeterioration, and Without the necessity of enclosing the same in an airtight compartment to prevent oxidation.

In accordance with my invention, I employ a carbonaceous resistanceelement around which is moulded refractory material, which is inert withrespect to the resistance element, and which has high heat conductivity.The refractory body, preferably, not only encloses the resistanceelement but also encloses a portion of the conductor terminals which areconnected to the resistance element. While the refractory body enclosesthe resistance element, special provision is made to prevent breaking ofthe resistance element or the enclosing sheath 5 due to differences inexpansion of the resistance element and sheath when both are heated.Enclosing the refractory body is a coating of fused material which formsa seal to prevent access of gas through the refractory body to thecarbonaceous resistance element, and in this manner prevents oxidationof the carbonaceous material when the resistor is operated in anoxidizing atmosphere.

In the preferred form of my invention, the

refractory material is formed of silicon carbide, recrystallized inwhole or in part, and the outer sealing coating is formed of fusedborax.

In making up the resistor, a graphite or other carbonaceous resistanceelement of the zigzag type or other suitable form is employed. The usualconducting terminals are secured to the resistance element and theelement and terminals are then coated with a 46 readily fusable orvolatilizable material such as paraffin wax. The assembled coatedresistance element and terminals are then placed in a mould whichconforms to the outside shape of the finished body. The space betweenthe. assembled resistor and mould is then rammed with the bodyrefractory. Any one of several mixtures may be employed. One suchmixture is composed of sand and coke in such proportions that, when theresistor is connected to a source of electrical energy to heat it to asuffioiently high temperature, silicon carbide is formed. Again theso-called carborundum firesand which is silicon carbide in the amorphousstate, may be used, and this material, when heated to a sufficientlyhigh temperature, will crystallize forming silicon carbide. I prefer,however, to use a body refractor'y mixture composed of crushed siliconcarbide grains because less time is occupied in preparing the resistorbody for use once the resistance element and terminals have been mouldedinto the body. \Vhen crushed silicon carbide grains are employed, Ipreferably employ a mixture of coarse and fine grains because suchmixture moulds better into the shape desired.

T o whatever mixture employed, I add 1% to 15% by weight of borax, andthen add enough Water so as to make the mixture of such consistency thatit can readily be rammed into the mould. The borax content depends uponthe use for which the resistor is designed. The thickness of the bodysurrounding the resistance element and terminals depends upon the usefor which the resistor is designed, and this thickness may vary betweena very thin layer such as to 1 or even thicker.

Having finished moulding the body around the resistance element andterminals, the

mould is removed from around the body and the whole is then dried out bypassing current through the resistor. As the body dries out, the energydissipated in the resistor is increased until the whole has attained awhite heat on the outside.

Upon cooling, the borax will be found to have melted forming a gas tightfilm on the outside of the body, and when the mixture, composed ofcrushed silicon carbide grains has been employed, it will be found ontransverse fracture that the silicon carbide next to the resistor isrecrystallized forming a solid or monolithic recrystallized siliconcarbide resistor sheath. Although recrystallization is not essential, itis preferable. During the heating operation, the wax coating on theresistance element will have melted and the wax seeped through the bodyof the sheath leaving a space between the resistance element and thesilicon carbide sheath. This space will eliminate any danger of crackingthe sheath or resistance element, due to difference in the expansion ofthe resistance element and sheath when the resistor is heated in use. Asthe silicon carbide sheath is an excellent conductor of heat, it permitsthe heat to be transmitted from the resistor to the furnace chamber witha minimum loss.

Another method of making one of these resistors complete with its caseor protection, is to mould the silicon carbide around the coatedresistance element and terminals as described above, but without theaddition of borax. Instead of borax, glue or someother temporary bond ismixed with the silicon carbide. The mould is then removed and theresistor is dried out at a temperature just suflicicnt to drive off themoisture used in moulding the body. When sufficiently dried, the wholeis then placed in a furnace and surrounded b a mixture of sand, coke andsawdust, simi ar to that used in making silicon carbide. The terminalsare then connected to a source of electrical energy and the resistor isrun at sufiicient energy consumption and length of time to recrystallizepart or the whole of the body. When the furnace is sufficiently cool, soas to permit the handling of the resistor and terminals as a unit, it isremoved and any adherent silicon carbide mixture is scraped off and thebody cleaned. A solution of borax and water is then poured over thebody. The body being porous, absorbs the solution readily. The properamount of borax being added in this manner the moisture is again drivenoff by heating carefully and slowly. When dried the resistor isconnected as before, to a source of electrical energy until a white heathas been attained, and then allowed to cool off. The resistor is nowready to be placed in any furnace adapted for its use.

. An advantage of this type of resistor is that no special chamber needbe built for the resistor so as to exclude oxygen. The resistor may beoperated in an atmosphere which is highly oxidizing withoutdeterioration. Furthermore, due to the strength of the heatshows partlyin elevation ing unit, comprising the resistorand termi nals, it may beplaced in any convenient position in the furnace. Electrical connectionto the resistor may be made outside the furnace and in case of failureor fracture, the resistance element, terminals and refractory may beremoved as a unit and readily replaced by a spare unit.

By the use of the zigzag type of carbona ceous heating element, it ispossible to have a resistor of large energy dissipating qualities,confined to a small space thus permitting high furnace temperatures,which may be obtained in a very short space of time. It will, however,be understood that if desired, other forms of carbonaceous resistanceelements ma be used.

Iii the accompanying drawings, Fig. 1

and partly in central longitudinal section, a heating unit constructedin accordance with one form of my invention, and Fig. 2 shows afragmentary transverse section of this unit taken on the line 2-2 ofFig. 1. In the drawing, 10 representsa zigzag graphite or othercarbonaceous resistance element to which is secured in any suitablemanner conducting terminals 11. The resistance element and terminals areembedded or incased in the refractory body material 14, which, as shown,envelops the entire outer surface of the resistance element. It does notpenetrate the slots in the zigzag element, and is slightly spaced fromthe outer surface of the resistance element and terminals as isindicated at 13 except at the outer ends of the terminals 11 where thesheath is in direct contact with the surface of the terminals. The outersurface of the refractory material 14 is sealed by a coating or film 15of fused borax. While the drawing shows the borax as forming a film onthe outside of the surface of the refractory material, it will beunderstood that borax may penetrate to some extent the refractorymaterial serving to fill the pores or interstices in the surface of the.refractory body.

I claim:

1. A resistor comprising a carbonaceous resistance element incased in amonolithic mass of silicon carbide, said mass being slightly spaced fromthe surface of said resistance element so as to independently expand andcontract with changes in temperature.

2. A heater unit comprising a carbonaceous resistance element, a siliconcarbide sheath enclosing said element, said sheath and element beingpositioned with a slight space therebetween so as to be free to expandand contract independently of one another.

8. A heating element consisting of a carbonaceous resistance element,terminals secured to said element and a monolithic silicon carbidesheath enclosing said element and terminals and being separate from saidresistance element so as to be free to expand and contract independentlyof said element and terminals.

4. A resistor comprising a carbonaceous resistance element incased in amonolithic mass of silicon carbide, the silicon carbide next to thesurface of the resistance element being recrystallized, said mass ofsilicon carbide being separate from said resistance element so as to befree to expand and contract independently of said resistance element.

5. A carbon resistance element, a silicon carbide sheath enclosing thesame and spaced therefrom sufficiently to permit relative expansion andcontraction of said element and sheath and fused borax at the outersurface of said sheath for rendering the sheath impervious to gas.

6. A heating element consisting of a carbon resistance element,conducting terminals secured thereto and a monolithic mass of siliconcarbide forming a sheath enclosing said element and terminals, said massbeing supported in contact with the terminals near their outer ends andbeing spaced from other portions of said terminals and from saidresistance element sufficiently to permit independent relative expansionand contraction of said mass and said element and terminals.

7. A heating element consisting of a carbon resistance element,conducting terminals secured thereto, a monolithic mass of siliconcarbide forming a sheath enclosing said element and terminals, said massbeing supported in contact with the terminals near their outer ends andbeing spaced from other portions of said terminals and from saidresistance element sufficiently to permit independent relative expansionand contraction of said mass and said element and terminals and fusedborax at the outer surface of said sheath for rendering said sheathimpervious to gas.

8. A method of forming a resistor which consists in coating a carbonresistance element with a readily fusible material, moulding around thecoated element a mixture of crushed silicon carbide grains and atemporary binder, slowly heating the resistance element by the passageof current therethrou h bide grains are converted into a monolithic massof recrystallized silicon carbide, which is in contact with the outer'ortions of the terminals and is spaced sufiiciently from the innerportions of said terminals and from the resistance element so as topermit relative expansion of said terminals and element and said mass ofrecr stallized silicon carbide, and whereby said orax forms a sealingmedium at the outer surface of said mass of recrystallized siliconcarbide.

JAMES KELLEHER.

until the entire mass has attained a whlte v heat.

9. A method of forming a heating element which consists in connectingconducting terminals to a zigzag graphite resistance element, coatingsaid resistance element and the inner portions of said terminals with aread- I ily fusible material without filling the slots between sectionsof the resistance element, moulding around said coated element andterminals a mixture of coarse and fine silicon carbide grains togetherwith a binder com-' prising borax, slowly heating the mass to a whiteheat by passing current through the resistance element whereby thesilicon car-

